An engine exhaust gas purification device

Abstract: An exhaust gas purification device comprising a three way catalyst (17), an oxidizing catalyst (20) and a NOx absorbent (23,24) which are arranged in this order in the exhaust passage. A rich air-fuel mixture is burned in the combustion chamber (3), and the exhaust gas discharged from the engine (1) is initially introduced into the three way catalyst (17). Secondary air is fed into the exhaust passage between the three way catalyst (17) and the oxidizing catalyst (20) so that the air-fuel ratio of the exhaust gas flowing into the oxidizing catalyst (20) and the NOx absorbent (23,24) becomes lean.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an engine exhaust gas purification device. 2. Description of the Related Art If a ratio of the total amount of air fed into the exhaust passage, the combustion chamber and the exhaust passage upstream of a certain position in the exhaust passage to the total amount of fuel fed into the exhaust passage, the combustion chamber and the exhaust passage upstream of the above-mentioned position in the exhaust passage is referred to as an air-fuel ratio of the exhaust gas at that certain position, in a known engine, a lean air-fuel mixture is burned in the combustion chamber, and a three way catalyst is arranged in the exhaust passage, and a NOx absorbent, which absorbs NOx when the air-fuel ratio of the exhaust gas flowing into the NOx absorbent is lean and releases absorbed NOx when the air-fuel ratio of the exhaust gas flowing into the NOx absorbent is rich, is arranged in the exhaust passage downstream of the three way catalyst (see PCT international publication WO93/07363). Namely, in an engine, the amount of production of NOx becomes maximum when an air-fuel mixture which is slightly leaner than the stoichiometric air-fuel ratio is burned, and the amount of production of NOx becomes smaller as the air-fuel ratio of air-fuel mixture is made leaner or richer with respect to the air-fuel ratio which is slightly leaner than the stoichiometric air-fuel ratio. In addition, when a lean air-fuel mixture is burned, the amount of unburned HC and CO discharged from the engine becomes considerably small, as compared with the case where a rich air-fuel mixture is burned. Accordingly, in the above-mentioned engine, the air-fuel ratio of the air-fuel mixture to be burned in the combustion chamber is made considerably lean to reduce the amount of NOx and unburned HC and CO discharged from the engine as much as possible. In addition, at this time, the NOx discharged from the engine is caused to be absorbed by the NOx absorbent as much as possible, and the unburned HC and CO discharged from the engine is caused to be oxidized by the three way catalyst as much as possible to reduce the amount of NOx and unburned HC and CO discharged into the outside air. Among the harmful components in the exhaust gas, NOx is a component which is the most difficult to purify and, if this NOx can be reduced by the three way catalyst, it is possible to considerably lower the amount of NOx in the exhaust gas. However, if the air-fuel ratio of the exhaust gas is lean, the three way catalyst does not carry out the reducing operation of NOx. Accordingly, if the air-fuel ratio of air-fuel mixture is made lean, i.e., the air-fuel ratio of the exhaust gas is made lean as in the above-mentioned engine, NOx discharged from the engine is not reduced by the three way catalyst and, accordingly, in the above-mentioned engine, NOx passes through the three way catalyst without being reduced and then flows into the NOx absorbent. Namely, in the above-mentioned engine, the first stage of the reducing operation of the amount of NOx is carried out by making the air-fuel ratio of the air-fuel mixture lean. In this case, it is true that the amount of NOx discharged from the engine is reduced due to this first stage of the reducing operation of the amount of NOx, but the absolute amount of NOx discharged from the engine is still large. Then, although NOx discharged from the engine flows into the three way catalyst, the amount of NOx is not reduced in the three way catalyst at all. Then, the NOx flows into the NOx absorbent, and the second stage of the reducing operation of the amount of NOx is carried out therein due to the absorbing operation of NOx by the NOx absorbent. In this way, in this engine, the reducing operation of NOx is carried out in two stages, but the main reducing operation of the amount of NOx relies on the absorbing operation of NOx by the NOx absorbent. However, even if the NOx absorbent is used, particularly when the absolute amount of NOx flowing into the NOx absorbent is large, all the NOx cannot always be absorbed in the NOx absorbent. Accordingly, it is difficult to make the amount of NOx, discharged into the outside air, almost zero as long as the main reducing operation of NOx is made to rely on only the absorbing operation of NOx by the NOx absorbent. SUMMARY OF THE INVENTION An object of the present invention is to provide an exhaust gas purification device capable of making the amount of NOx, discharged into the atmosphere, almost zero. According to the present invention, there is provided an engine exhaust gas purification device, comprising: air-fuel mixture control means for making an air-fuel mixture burned in a combustion chamber of the engine a rich air-fuel mixture; a three way catalyst arranged in an exhaust passage of the engine; a NOx absorbent arranged in the exhaust passage downstream of the three way catalyst, the NOx absorbent absorbing NOx therein when an air-fuel ratio of exhaust gas flowing into the NOx absorbent is lean, secondary air supply means for feeding secondary air into the exhaust passage between the three way catalyst and the NOx absorbent; and secondary air control means for controlling an amount of secondary air fed from the secondary air supply means to maintain the air-fuel ratio of the exhaust gas flowing into the NOx absorbent at a lean air-fuel ratio. The present invention may be more fully understood from the description of preferred embodiments of the invention set forth below, together with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS In the drawings: Fig. 1 is a general view of an engine; Fig. 2 is a cross-sectional view of the electrically heated three way catalyst and the electrically heated oxidizing catalyst; Fig. 3 is a diagram illustrating the output of the air-fuel ratio sensor; Fig. 4 is a diagram illustrating the concentration of NOx and unburned HC and CO discharged from the engine; Fig. 5 is a diagram illustrating the purification rate in the three way catalyst; Fig. 6 is a diagram illustrating the purification rate in the oxidizing catalyst; Figs. 7A and 7B are diagrams illustrating the absorbing and releasing operation of NOx; Fig. 8 is a diagram illustrating the absorption rate of NOx; Fig. 9 is a diagram illustrating the air-fuel ratio at various points in the exhaust passage after the warm-up of the engine is completed; Fig. 10 is a diagram illustrating the air-fuel ratio at various points in the exhaust passage before the warm-up of the engine is completed; Figs. 11A and 11B are diagrams illustrating the control region of the air-fuel ratio; Fig. 12 is a time chart illustrating the control of the electrical heating operation, etc., at the time of the start of the engine; Fig. 13 is a flow chart of the control routine of the engine; Fig. 14 is a flow chart for calculating the fuel injection time TAU; Fig. 15 is a flow chart for calculating the duty ratio DUTY2; Fig. 16 is a flow chart for calculating the duty ratio DUTY1; Figs. 17A, 17B and 17C are views illustrating the ignition time , etc.; Fig. 18 is a general view of another embodiment of the engine; Fig. 19 is a diagram illustrating the air-fuel ratio at various points in the exhaust passage at the time after the warm-up of the engine is completed; Fig. 20 is a diagram illustrating the air-fuel ratio at various points in the exhaust passage at the time before the warm-up of the engine is completed; Fig. 21 is a diagram illustrating the air-fuel ratio at various points in the exhaust passage at the time when NOx is released from the NOx absorbent; Fig. 22 is a flow chart of the control routine of the engine; Fig. 23 is a flow chart of the control routine of the engine; Fig. 24 is a flow chart for calculating the fuel injection time TAU; Fig. 25 is a flow chart for calculating the duty ratio DUTY2; Fig. 26 is a flow chart for calculating the duty ratio DUTY1; and Fig. 27 is a flow chart for calculating the duty ratio DUTY3. DESCRIPTION OF PREFERRED EMBODIMENTS Referring to Figure 1, reference numeral 1 designates an engine body, 2 a piston, 3 a combustion chamber, and 4 a spark plug; 5 designates an intake valve, 6 an intake port, 7 an exhaust valve, and 8 an exhaust port. The intake ports 6 are connected to a surge tank 10 via corresponding intake branch pipes 9, and fuel injectors 11 are arranged in the intake branch pipes 9 to inject fuel toward the corresponding intake ports 6. The surge tank 10 is connected to an air cleaner 13 via an intake duct 12, and a throttle valve 14 is arranged.in the intake duct 12. In addition, a mass flow detector 15 which is able to detect the mass flow amount of air fed into the engine cylinders is arranged in the intake duct 12 upstream of the throttle valve 14. The exhaust ports 8 are connected, via an exhaust manifold 16, to a catalytic converter 18 including an electrically heated three way catalyst 17 therein, and the outlet of the catalytic converter 18 is connected, via an exhaust pipe 19, to a catalytic converter 21 including an electrically heated oxidizing catalyst 20 therein. In addition, the outlet of the catalytic converter 21 is connected, via an exhaust pipe 22, to a casing 25 including a pair of NOx absorbents 23, 24 therein. As illustrated in Fig. 1, the engine has a secondary air supply device which comprises an air pump 26 driven by the engine, a first secondary air supply valve 27 and a second secondary air supply valve 28. The air suction port of the air pump 26 is connected to the intake passage located between the air cleaner 13 and the mass flow detector 15 via a conduit 29. Conversely, the air discharge port of the air pump 26 is connected, on the one hand, to the exhaust manifold 16 via a conduit 30 and the first secondary air supply valve 27 and, on the other hand, to the exhaust pipe 19 via the conduit 30 and the second secondary air supply valve 28. In addition, a first air-fuel ratio sensor 31 is arranged in the exhaust manifold 16 upstream of the inlet of secondary air flowing into the exhaust manifold 16 from the first secondary air supply valve 27, and a second air-fuel ratio sensor 32 is arranged in the exhaust pipe 19 downstream of the inlet of secondary air flowing into the exhaust pipe 19 from the second secondary air supply valve 28. Furthermore, a temperature sensor 33 is attached to the engine body 1. Both the electrically heated three way catalyst 17 and the electrically heated oxidizing catalyst 20 have a construction such that, for example, a metallic thin plate 34 and a metallic corrugated plate 35 are alternately wound about a common axis as illustrated in Fig. 2, and catalytic particles are carried on the metallic thin plate 34 and the metallic corrugated plate 35. In addition, by causing electric current to flow within the metallic thin plate 34 and the metallic corrugated plate 35, these plates 34 and 35 are heated, and thus, the catalytic particles carried on these plates 34 and 35 are heated. Accordingly, the metallic thin plate 34 and the metallic corrugated plate 35 construct a carrier for catalyst and, at the same time, functions as a heater. This heater is controlled by signals output from an electronic control unit 40. The electronic control unit 40 is constructed as a digital computer and comprises a ROM (read only memory) 42, a RAM (random access memory) 43, a CPU (microprocessor etc.) 44, a back-up RAM45 continuously connected to the power source, an input port 46 and an output port 47. The mass flow detector 15 produces an output voltage which is proportional to the mass flow amount of air fed into the engine cylinders, and this output voltage is input into the input port 46 via a corresponding AD converter 48. In addition, the air-fuel ratio sensors 31 and 32 produce an output voltage V which corresponds to the air-fuel ratio of exhaust gas A/F, as illustrated in Fig. 3, and thus, the air-fuel ratio of exhaust gas A/F can be found from the output voltage V. The output voltages V of the air-fuel ratio sensors 31, 32 are input into the input port 46 via corresponding AD converters 48. The temperature sensor 33 produces an output voltage which is proportional to the temperature of the cooling water of the engine, and this output voltage is input into the input port 46 via a corresponding AD converter 48. An engine speed sensor 36 which produces output pulses representing the engine speed is connected to the input port 46 and, in addition, the operation signal of a starter switch 37 for driving the starter motor is input into the input port 46. Furthermore, the operation signals of a preheat switch 38 for controlling the supply of power to the electrically heated three way catalyst 17 and the electrically heated oxidizing catalyst 20 are input into the input port 46. The output port 47 is connected, via corresponding drive circuits 49, to a spark plug 4, fuel injectors 11, a relay 50 for controlling the supply of power to the electrically heated three way catalyst 17, a relay 51 for controlling the supply of power to the electrically heated oxidizing catalyst 20, and the secondary air supply valves 27 and 28. The amount of secondary air fed from the secondary air supply valves 27 and 28 is controlled by controlling a ratio of the time period of opening of the valve per a unit time, i.e., by controlling a duty ratio, and the amount of secondary air fed into the exhaust manifold 16 and the exhaust pipe 19 from the secondary air supply valves 27 and 28, respectively, is increased as the duty ratio becomes larger. Figure 4 illustrates the relationship between the air-fuel ratio A/F of the air-fuel mixture burned in the combustion chamber 3 and the concentration of NOx and unburned HC and CO produced in the combustion chamber 3, i.e., the concentration of NOx and unburned HC and CO discharged from the engine. As illustrated in Fig. 4, the amount of NOx discharged from the engine becomes maximum when the air-fuel ratio of air-fuel mixture A/F is slightly leaner than the stoichiometric air-fuel ratio, and the amount of NOx discharged from the engine is reduced as the air-fuel ratio of air-fuel mixture A/F is made leaner or richer with respect to the air-fuel ratio of air-fuel ratio A/F at which the amount of NOx discharged from the engine becomes maximum. Conversely, the amount of unburned HC and CO discharged from the engine is increased as the air-fuel ratio of air-fuel mixture A/F becomes richer. Figure 5 illustrates the relationship between the air-fuel ratio A/F of the exhaust gas flowing into the three way catalyst 17 and the purification rate of NOx and unburned HC and CO by the three way catalyst 17. When the air-fuel ratio A/F of the exhaust gas flowing into the three way catalyst 17 is lean, the reducing operation of NOx by the three way catalyst 17 is not carried out and thus, as illustrated by the solid line in Fig. 5, when the air-fuel ratio A/F of the exhaust gas flowing into the three way catalyst 17 become lean, the purification rate of NOx abruptly drops. Conversely, when the air-fuel ratio A/F of the exhaust gas flowing into the three way catalyst 17 is rich, the purification rate of NOx becomes almost 100 percent unless the air-fuel ratio of exhaust gas A/F becomes excessively rich. In addition, as illustrated by the broken line in Fig. 5, the purification rate of unburned HC and CO becomes maximum when the air-fuel ratio A/F of the exhaust gas flowing into the three way catalyst 17 is slightly leaner than the stoichiometric air-fuel ratio. Conversely, if the air-fuel ratio A/F of the exhaust gas flowing into the three way catalyst 17 is made still leaner, the purification rate of unburned HC and CO gradually drops as the air-fuel ratio of exhaust gas A/F becomes leaner and, if the air-fuel ratio A/F of the exhaust gas flowing into the three way catalyst 17 is made rich, the purification rate of unburned HC and CO abruptly drops as the air-fuel ratio of the exhaust gas A/F becomes rich. Figure 6 illustrates the relationship between the air-fuel ratio A/F of the exhaust gas flowing into the oxidizing catalyst 20 and the purification rate of unburned HC and CO by the oxidizing catalyst 20. As illustrated in Fig. 6, the reduction rate of unburned HC and CO becomes maximum when the air-fuel ratio A/F of the exhaust gas flowing into the oxidizing catalyst 20 is slightly leaner than the stoichiometric air-fuel ratio. Conversely, if the air-fuel ratio A/F of the exhaust gas flowing into the oxidizing catalyst 20 is made still leaner, the purification rate of unburned HC and CO gradually drops as the air-fuel ratio of exhaust gas A/F becomes leaner and, if the air-fuel ratio A/F of the exhaust gas flowing into the oxidizing catalyst 20 is made rich, the purification rate of unburned HC and CO abruptly drops as the air-fuel ratio of exhaust gas A/F becomes leaner. Next, the NOx absorbents 23, 24 arranged downstream of both the three way catalyst 17 and the oxidizing catalyst 20 will be described. These NOx absorbents 23, 24 use, for example, alumina as a carrier. On this carrier, at least one substance selected from alkali metals, for example, potassium K, sodium Na, lithium Li, and cesium Cs; alkali-earth metals, for example, barium Ba and calcium Ca; rare-earth metals, for example, lanthanum La and yttrium Y; transition metals such as iron Fe and precious metals such as platinum Pt is carried. These NOx absorbents 23, 24 perform the absorption and releasing operation of NOx by absorbing the NOx when the air-fuel ratio of the exhaust gas flowing into the NOx absorbents 23, 24 is lean, while releasing the absorbed NOx when the concentration of oxygen in the exhaust gas flowing into the NOx absorbents 23, 24 falls. When these NOx absorbents 23, 24 are disposed in the exhaust passage of the engine, these NOx absorbents 23, 24 actually perform the absorption and releasing operation of NOx, but there are areas of the exact mechanism of this absorption and releasing operation which are not clear. However, it can be considered that this absorption and releasing operation is conducted by the mechanism as shown in Figs. 7A and 7B. This mechanism will be explained by using as an example a case where platinum Pt and barium Ba are carried on the carrier, but a similar mechanism is obtained even if another precious metal, alkali metal, alkali earth metal, rare earth metal or transition metal is used. Namely, if the air-fuel ratio of the exhaust gas flowing into the NOx absorbents 23, 24 becomes lean, i.e., the concentration of oxygen in the exhaust gas is greatly increased, as shown in Fig. 7A, the oxygen O 2 is deposited on the surface of the platinum Pt in the form of O 2 - or O 2- . The NO in the exhaust gas reacts with the O 2 - or O 2- on the surface of the platinum Pt and becomes NO 2 (2NO + O 2 → 2NO 2 ). Subsequently, a part of the produced NO 2 is oxidized on the platinum Pt and absorbed into the absorbent. While bonding with the barium oxide BaO, it is diffused in the absorbent in the form of nitric acid ions NO 3 - as shown in Fig. 7A. In this way, NOx is absorbed into the NOx absorbents 23, 24. Contrary to this, when the oxygen concentration in the exhaust gas flowing into the NOx absorbents 23, 24 is lowered and the production of NO 2 is lowered, the reaction proceeds in an inverse direction (NO 3 - → NO 2 ), and thus nitric acid ions NO 3 - in the absorbent are released in the form of NO 2 from the absorbent. Namely, when the oxygen concentration in the exhaust gas flowing into the NOx absorbents 23, 24 is lowered, for example when the air-fuel ratio of the exhaust gas flowing into the NOx absorbents 23, 24 is switched over from lean to rich, NOx is released from the NOx absorbents 23, 24. Accordingly, where it is desirable that NOx is not released from the NOx absorbents 23, 24, it is necessary to maintain the air-fuel ratio of the exhaust gas flowing into the NOx absorbents 23, 24 at a lean air-fuel ratio. Although the NOx absorbents 23, 24 perform the absorbing operation of NOx as mentioned above, the absorption rate of NOx of the NOx absorbents 23, 24 has temperature characteristics which change in accordance with kinds of metals carried on the carrier. Namely, as illustrated in Fig. 8, in the Pt-Ba absorbent formed by the combination of platinum Pt and barium Ba, the absorption rate of NOx becomes maximum in a middle temperature ranging from 200°C to 500°C. Conversely, in the Pt-Ba-Fe absorbent formed by the combination of platinum Pt, barium Ba and iron Fe, the absorption rate of NOx becomes maximum in a low temperature below 200°C. For example, in the Pt-Ba absorbent, if the temperature of the absorbent becomes low, the oxidizing reaction of NOx on the surface of platinum Pt does not proceed and, in addition, the absorption speed of NOx into the absorbent becomes low. As a result, the absorption rate of NOx drops. Conversely, if the temperature of the absorbent becomes high, nitrite in the absorbent is dissolved, and NOx is released from the absorbent. As a result, the absorption rate of NOx drops. However, the relationships between the temperature and the oxidizing reaction of NOx, between the temperature and the absorbing operation of NOx and between the temperature and the dissolving operation of nitrite change in accordance with the kinds of metals carried on the carrier. Accordingly, as illustrated in Fig. 8, the temperature regions in which the maximum absorption rate of NOx is obtained change in accordance with the kinds of metals carried on the carrier. In this way, since the temperature regions in which the maximum absorption rate of NOx is obtained change in accordance with the kinds of metals carried on the carrier, it is preferable to use the NOx adsorbents by taking this matter into consideration. Thus, in the embodiment illustrated in Fig. 1, a NOx absorbent such as the Pt-Ba-Fe absorbent having the maximum absorption rate of NOx at a low temperature is used as the NOx absorbent 23, and a NOx absorbent such as Pt-Ba absorbent having the maximum absorption rate of NOx at a middle temperature is used as the NOx absorbent 24. In this way, if the NOx adsorbents 23 and 24 having different temperature regions in which the absorption rate of NOx becomes maximum are arranged in series, NOx can be absorbed in the NOx adsorbents 23, 24 over a wide range of the operating state of the engine from the start of the engine at which the temperature of the exhaust gas is low to the high load operating state in which the temperature of the exhaust gas becomes high. Figure 9 schematically illustrates the air-fuel ratio A/F of the air-fuel mixture burned in the combustion chamber 3 and the air-fuel ratio A/F of the exhaust gas in the exhaust passage after the warm-up of the engine is completed. Namely, what should be initially done to reduce the amount of NOx discharged into the outside air is to reduce the amount of NOx produced in the combustion chamber 3. To this end, as can be seen from Fig. 4, it is necessary to make the air-fuel ratio A/F of the air-fuel mixture burned in the combustion chamber 3 either rich or extremely lean. In this case, however, taking the purifying operation of NOx by the three way catalyst 17 into consideration, as can be seen from Fig. 5, the air-fuel ratio A/F of the exhaust gas flowing into the three way catalyst 17 must be rich. Accordingly, in the present invention, the air-fuel ratio A/F of the air-fuel mixture burned in the combustion chamber 3 is made rich. In this case, however, as can be seen from Fig. 4, the amount of unburned HC and CO produced in the combustion chamber 3 is increased as the air-fuel ratio of air-fuel mixture A/F becomes rich and, as can be seen from Fig. 5, the purification rate of unburned HC and CO by the three way catalyst 17 drops as the air-fuel ratio of air-fuel mixture A/F becomes rich. Accordingly, taking the reduction in the amount of unburned HC and CO into consideration in addition to the reduction in the amount of NOx, it is preferable to bring the air-fuel ratio of air-fuel mixture A/F as close to the stoichiometric air-fuel ratio as possible. However, if bringing the air-fuel ratio of air-fuel mixture A/F too close to the stoichiometric air-fuel ratio, the air-fuel ratio of air-fuel mixture A/F becomes lean due to a slight change in the amount of fuel or air. Nevertheless, as can be seen from Figs. 4 and 5, if the air-fuel ratio of air-fuel mixture A/F becomes lean, the amount of NOx flowing out from the three way catalyst 17 without being purified is extremely increased. Accordingly, it is impossible to bring the air-fuel ratio of air-fuel mixture A/F too close to the stoichiometric. Thus, in the embodiment according to the present invention, to obtain a high purification rate of NOx and as high a purification rate of unburned HC and CO as possible, the air-fuel ratio of air-fuel mixture A/F is maintained at a rich air-fuel ratio which is as close to the stoichiometric air-fuel ratio as possible within a range in which the air-fuel ratio of air-fuel mixture does not become lean due to a slight change in the amount of fuel and air. Specifically speaking, in the embodiment according to the present invention, the air-fuel ratio of air-fuel mixture A/F is controlled by feedback operation on the basis of the output signal of the first air-fuel ratio sensor 31 so that the air-fuel ratio of air-fuel mixture A/F is maintained between about 14.3 and about 14.5, as illustrated in Fig. 11A. Note that, when this feedback control is carried out, the purification rate of NOx becomes almost 99.9 percent, and the purification rate of unburned HC and CO becomes more than 80 percent. Accordingly, a small amount of unburned HC and CO is contained in the exhaust gas passing through the three way catalyst 17, but the amount of NOx contained in this exhaust gas becomes extremely small. The exhaust gas passing through the three way catalyst 17 flows into the oxidizing catalyst 20 and then flows into the NOx adsorbents 23, 24. In this case, as illustrated in Fig. 6, the purification rate of unburned HC and CO by the oxidizing catalyst 20 becomes the maximum when the air-fuel ratio A/F of the exhaust gas flowing into the oxidizing catalyst 20 is slightly leaner than the stoichiometric air-fuel ratio and, if the air-fuel ratio A/F of the exhaust gas flowing into the oxidizing catalyst 20 becomes rich, the purification rate of unburned HC and CO abruptly drops. In addition, to have NOx absorbed in the NOx absorbents 23, 24, the air-fuel ratio A/F of the exhaust gas flowing into the NOx adsorbents 23, 24 must be made lean and, if the air-fuel ratio A/F of the exhaust gas flowing into the NOx absorbents 23, 24 is made rich, NOx is released from the NOx absorbents 23, 24. Accordingly, to sufficiently purify unburned HC and CO in the exhaust gas passing through the three way catalyst 17 by the oxidizing catalyst 20 and cause NOx in the exhaust gas passing through the three way catalyst 17 to be absorbed in the NOx absorbents 23, 24, the air-fuel ratio A/F of the exhaust gas flowing into both the oxidizing catalyst 20 and the NOx absorbents 23, 24 must be maintained at a lean air-fuel ratio. To this end, as illustrated in Fig. 9, secondary air is fed into the exhaust pipe 19 upstream of the oxidizing catalyst 20 and the NOx absorbents 23, 24. As mentioned above, to make the purification rate of unburned HC and CO by the oxidizing catalyst 20 the maximum, the air-fuel ratio of exhaust gas A/F must be maintained at an air-fuel ratio which is slightly leaner than the stoichiometric air-fuel ratio. Namely, at this time, even if the amount of fuel, air fed into the engine cylinders, or secondary air slightly changes, the air-fuel ratio of exhaust gas A/F must not become rich. Accordingly, in the embodiment according to the present invention, the amount of secondary air fed into the exhaust pipe 19 is controlled by feedback operation on the basis of the output signal of the second air-fuel ratio sensor 32 so that the air-fuel ratio of exhaust gas A/F is maintained between about 15.1 and about 15.3, as illustrated in Fig. 11B. Note that, when this feedback control is carried out, the purification rate of unburned HC and CO becomes almost 100 percent and, since the amount of NOx in the exhaust gas passing through the three way catalyst 17 is extremely small, almost all the NOx in the exhaust gas passing through the three way catalyst 17 is absorbed in the NOx absorbents 23, 24. Accordingly, in the embodiment according to the present invention, the amount of NOx and unburned HC and CO discharged into the outside air becomes almost zero. As mentioned above, the amount of NOx in the exhaust gas passing through the three way catalyst 17 is extremely small. Accordingly, if the NOx absorbents 23, 24 are constructed so that they have a large volume, it is thought that the absorbing ability of NOx of the NOx absorbents 23, 24 will not be saturated within a time period in which a motor vehicle can be used. Accordingly, in the embodiment illustrated in Fig. 9, basically, it is not necessary to exchange the NOx absorbents 23, 24 with new ones. However, where it is difficult to sufficiently increase the volume of the NOx absorbents 23, 24, it is necessary to periodically exchange the NOx absorbents 23, 24 with new ones. Figure 10 schematically illustrates the air-fuel ratio A/F of the air-fuel mixture burned in the combustion chamber 3 and the air-fuel ratio A/F of the exhaust gas in the exhaust passage at the time before the warm-up of the engine is completed. At the time of warm-up operation of the engine wherein the temperature of the engine is low, the air-fuel ratio A/F of the air-fuel mixture burned in the combustion chamber 3 is made a rich air-fuel ration ranging from about 13.1 to about 14.0 and, accordingly, at this time, as can be seen from Fig. 10, the air-fuel ratio A/F of the exhaust gas discharged from the engine becomes a rich air-fuel ratio between about 13.0 and about 14.0. At this time, however, to obtain the purification rate of NOx and unburned HC and CO, which is almost the same as the purification rate obtained after the completion of warm-up of the engine, it is necessary to make the air-fuel ratio A/F of the exhaust gas flowing into the three way catalyst 17 a rich air-fuel ratio ranging from about 14.3 to about 14.5 and, to this end, in the embodiment according to the present invention, as illustrated in Fig. 10, secondary air is fed into the exhaust manifold 16 upstream of three way catalyst 17. In addition, at this time, in the same manner as carried out after the completion of the warm-up of the engine, secondary air is also fed into the exhaust pipe 19 so that the air-fuel ratio of the exhaust gas flowing into the oxidizing catalyst 20 and the NOx absorbents 23, 24 becomes a lean air-fuel ratio ranging from about 15.2 to about 15.3. The air-fuel ratio sensors 31, 32 do not produce a regular output signal unless the temperature thereof exceeds a certain fixed temperature. Accordingly, the amount of secondary air fed into the exhaust manifold 16 and the exhaust pipe 19 is determined based on the target air-fuel ratio of the air-fuel mixture until before the air-fuel ratio sensors 31, 32 produce a regular output signal, and the amount of secondary air fed into the exhaust manifold 16 and the exhaust pipe 19 is controlled by feedback operation on the basis of the output signals of the air-fuel ratio sensors 31, 32 after the air-fuel ratio sensors 31, 32 produce a regular output signal. As can be seen from Fig. 8, even if the temperature of the NOx absorbent 23 is low, the NOx absorbing operation by the NOx absorbent 23 is started, and accordingly, if the engine is started, the NOx absorbing operation of the NOx absorbent 23 is immediately started. However, the purifying operation of NOx and unburned HC and CO by the three way catalyst 17 is not started unless the temperature of the three way catalyst 17 becomes relatively high and, similarly, the purifying operation of unburned HC and CO by the oxidizing catalyst 20 is not started unless the temperature of the oxidizing catalyst 20 becomes relatively high. Accordingly, to immediately start the purifying operation of NOx and unburned HC and CO by the three way catalyst 17 and the purifying operation of unburned HC and CO by the oxidizing catalyst 20 after the engine is started, it is necessary to increase the temperature of the three way catalyst 17 and the oxidizing catalyst 20 to some extent before the start of the engine. To this end, in the embodiment according to the present invention, the three way catalyst 17 and the oxidizing catalyst 20 are electrically heated before the start of the engine. For example, as illustrated in Fig. 12, when the engine is started, initially, the preheat switch 38 is turned ON by a driver and thereby, the supply of power to the heaters, i.e., the supply of power to the three way catalyst 17 and the oxidizing catalyst 20 is started. Then, shortly after that, the starter switch 37 is turned ON by the driver, and thereby the engine is started. If the engine is started, the supply of secondary air is immediately started. If this method illustrated in Fig. 12 is carried out, it is possible to start the purifying operation of NOx and unburned HC and CO by the three way catalyst 17 as well as the purifying operation of unburned HC and CO by the oxidizing catalyst 20 from the time of the start of the engine. In this case, however, to carry out this method as illustrated in Fig. 12, the driver must be obliged to turn the starter switch 37 ON after some time has elapsed after the preheat switch 38 is turned ON. In this case, however, to be sure to have a driver observe this obligation, a special system may be adopted. For example, one may adopt a system such that, when a fixed time has not elapsed after the preheat switch 38 is turned ON, even if the starter switch 37 is turned ON, the starter motor is not driven. In addition, one may adopt another system such that, the temperatures of the three way catalyst 17 and the oxidizing catalyst 20 are detected, and the starter motor can be driven after the temperature of the three way catalyst 17 and the oxidizing catalyst 20 sufficiently raise. In addition, in the embodiment according to the present invention, as illustrated in Fig. 12, the ignition time is retarded for a short time after the engine is started. If the ignition time is retarded, since the combustion is prolonged to the end of the power stroke, the temperature of the exhaust gas increases. If the temperature of the exhaust gas increases, the three way catalyst 17 and the oxidizing catalyst 20 are maintained at a high temperature due to the heating of the exhaust gas, and thus, it is possible to obtain a good purifying operation of NOx and unburned HC and CO by the three way catalyst 17 and a good purifying operation of unburned HC and CO by the oxidizing catalyst 20. Next, the control routine for the engine will be described with reference to Figs. 13 through 16. Referring to Fig. 13, in step 100, it is determined based on, for example, the temperature of the engine coolant whether or not the warm-up of the engine is completed. If the warm-up of the engine has been completed, the routine goes to step 101, and the first secondary air supply valve 27 is closed. Accordingly, the supply of secondary air into the exhaust manifold 16 is stopped. Then, in step 102, the fuel injection time TAU is calculated. The calculation routine of this fuel injection time TAU is shown in Fig. 14. Referring to Fig. 14, in step 120, it is determined whether or not the air-fuel ratio of exhaust gas A/F detected by the first air-fuel ratio sensor 31 is smaller than 14.5. If A/F ≧ 14.5, the routine goes to step 123, and a fixed value A is added to the correction coefficient FAFR. Then, the routine goes to step 124. Conversely, if A/F < 14.5, the routine goes to step 121, and it is determined whether or not the air-fuel ratio of exhaust gas A/F detected by the first air-fuel ratio sensor 31 is larger than 14.3. If A/F ≦ 14.3, the routine goes to step 122, and a fixed value B is subtracted from the correction coefficient FAFR. Then, the routine goes to step 124. Conversely, if A/F > 14.3, the routine goes to step 124. In step 124, the fuel injection time TAU is calculated based on the following formula. TAU = K•Q/N•FAFR Where, Q represents the mass flow amount of air detected by the mass flow detector 15, and N represents an engine speed. Accordingly, Q/N represents the mass flow amount of air fed into the engine cylinder per one cycle. In addition, K is a constant value and is determined so that, when the amount of injected fuel is made K·Q/N, the air-fuel ratio of the air-fuel mixture fed into the engine cylinders becomes equal to the stoichiometric air-fuel ratio. In other words, K·Q/N represents a basic fuel injection time which is necessary to make the air-fuel ratio of air-fuel mixture the stoichiometric air-fuel ratio, and accordingly, the fuel injection time TAU is found by multiplying the basic fuel injection time K·Q/N by the correction coefficient FAFR. As can be seen from Fig. 14, if A/F ≧ 14.5, the correction coefficient FAFR is increased and, if A/F ≦ 14.3, the correction coefficient FAFR is decreased. In addition, if 14.3 < A/F < 14.5, the correction coefficient FAFR is maintained as it is. Accordingly, the air-fuel ratio A/F of the air-fuel mixture fed into the combustion chamber 3 is maintained between 14.3 and 14.5. Turning to Fig. 13, if the calculation of the fuel injection time TAU is completed in step 102, the routine goes to step 103, and the duty ratio DUTY2 for controlling the second secondary air supply valve 28 is calculated. The calculation routine of this duty ratio DUTY2 is shown in Fig. 15. Referring to Fig. 15, in step 130, it is determined whether or not the air-fuel ratio of exhaust gas A/F detected by the second air-fuel ratio sensor 32 is larger than 15.1. If A/F ≦ 15.1, the routine goes to step 133, and a fixed value C is added to the duty ratio DUTY2. Conversely, if A/F > 15.1, the routine goes to step 131. In step 131, it is determined whether or not the air-fuel ratio of exhaust gas A/F detected by the second air-fuel ratio sensor 32 is smaller than 15.3. If A/F ≧ 15.3, the routine goes to step 132, and a fixed value D is subtracted from the duty ratio DUTY2. As can be seen from Fig. 15, when A/F ≦ 15.1, the duty ratio DUTY2 is increased, and thereby the amount of secondary air fed into the exhaust pipe 19 from the second secondary air supply valve 28 is increased. Conversely, when A/F ≧ 15.3, the duty ratio DUTY2 is decreased, and thereby the amount of secondary air fed into the exhaust pipe 19 from the second secondary air supply valve 28 is reduced. In addition, when 15.1 < A/F < 15.3, the duty ratio DUTY2 is maintained as it is. Thus, the air-fuel ratio A/F of the exhaust gas flowing into the oxidizing catalyst 20 and the NOx absorbents 23, 24 is maintained between 15.1 and 15.3. Turning to Fig. 13, if the duty ratio DUTY2 is calculated in step 103, the routine goes to step 104, and the ignition time  is calculated. This ignition time  is stored in advance in the ROM42 in the form of a map as illustrated in Fig. 17A as a function of the engine speed N and the engine load Q/N (mass flow amount of air Q fed into the engine cylinders/the engine speed N). Then, is step 105, it is determined whether or not a fixed time has elapsed after the engine is started. If the fixed time has elapsed after the engine is started, the processing cycle is completed. Conversely, if the fixed time has not elapsed yet after the engine is started, the routine goes to step 106, and the ignition time  is retarded by α. When it is determined in step 100 that the warm-up of the engine is not completed, the routine goes to step 107, and the correction coefficient KR for the basic fuel injection time K·Q/N is calculated. This correction coefficient KR is a function of, for example, the temperature Tw of the cooling water of the engine and is increased as the temperature Tw of the cooling water of the engine becomes low, as illustrated in Fig. 17B. Then, in step 108, the fuel injection time TAU is calculated by multiplying the basis fuel injection time K·Q/N by the correction coefficient KR. At this time, the air-fuel ratio A/F of the air-fuel mixture fed into the combustion chamber 3 is made a rich air-fuel ratio corresponding to the value of the correction coefficient KR. Then, in step 109, it is determined whether or not the feedback control by the air-fuel ratio sensors 31, 32 becomes possible. If the feedback control by the air-fuel ratio sensors 31, 32 is not possible, the routine goes to step 110, and the duty ratio DUTY1 for controlling the first secondary air supply valve 27 is calculated. This duty ratio DUTY1 is a duty ratio which is necessary to bring the air-fuel ratio A/F of the exhaust gas flowing into the three way catalyst 17 to an air-fuel ratio ranging from about 14.3 to about 14.5, and this duty ratio DUTY1 is stored in advance in the ROM42 in the form of a map as illustrated in Fig. 17C as a function of, for example, the mass flow amount Q of air and the correction coefficient KR. Then, in step 111, the duty ratio DUTY2 for controlling the second secondary air supply valve 28 is calculated. This duty ratio DUTY2 is a duty ratio which is necessary to bring the air-fuel ratio A/F of the exhaust gas flowing into the oxidizing catalyst 20 and the NOx absorbents 23, 24 to an air-fuel ratio ranging from about 15.1 to about 15.3. This duty ratio DUTY2 is stored in advance in the ROM42 in the form of a map as illustrated in Fig. 17C as a function of, for example, the mass flow amount Q of air and the correction coefficient KR, or as a function of only the mass flow amount Q of air When it is determined in step 109 that the feedback control by the air-fuel ratio sensors 31, 32 is possible, the routine goes to step 112, and the duty ratio DUTY1 for controlling the first secondary air supply valve 27 is calculated. The calculation routine of this duty ratio DUTY1 is shown in Fig. 16. Referring to Fig. 16, in step 140, it is determined whether or not the air-fuel ratio of exhaust A/F detected by the first air-fuel ratio sensor 31 is larger than 14.3. If A/F ≦ 14.3, the routine goes to step 143, and a fixed value E is added to the duty ratio DUTY1. Conversely, if A/F > 14.3, the routine goes to step 141, and it is determined whether or not the air-fuel ratio of exhaust gas A/F detected by the first air-fuel ratio sensor 31 is smaller than 14.5. If A/F ≧ 14.5, the routine goes to step 142, and a fixed value F is subtracted from the duty ratio DUTY1. As can been seen from Fig. 16, if A/F ≦ 14.3, the duty ratio DUTY1 is increased, and thereby, the amount of secondary air fed into the exhaust manifold 16 from the first secondary air supply valve 27 is increased. Conversely, if A/F ≧ 14.5, the duty ratio DUTY1 is decreased, and thereby, the amount of secondary air fed into the exhaust manifold 16 from the first secondary air supply valve 27 is reduced. In addition, if 14.3 < A/F < 14.5, the duty ratio DUTY1 is maintained as it is. Thus, the air-fuel ratio A/F of the exhaust gas flowing into the three way catalyst 17 is maintained between 14.3 and 14.5 Then, in step 103, the amount of secondary air fed from the second secondary air supply valve 28 is controlled by feedback on the basis of the output signal of the second air-fuel ratio sensor 32 so that the air-fuel ratio A/F of the exhaust gas flowing into the oxidizing catalyst 20 and the NOx absorbents 23, 24 is maintained between 15.1 and 15.3. Figure 18 illustrates a second embodiment. In Fig. 18, similar components are indicated with the same reference numerals used in Fig. 1. This second embodiment is different from the first embodiment illustrated in Fig. 1 in the following three points. Namely, first, the order of the arrangement of the oxidizing catalyst 20 and the NOx absorbents 23, 24 is reversed compared to the first embodiment. Namely, in the second embodiment, the outlet of the catalytic converter 18 including the electrically heated three way catalyst 17 therein is connected, via the exhaust pipe 19, to the causing 25 including a pair of the Nox absorbents 23, 24 therein, and the outlet of the casing 25 is connected to the catalytic converter 21 including the electrically heated oxidizing catalyst 20 therein. Secondly, in the second embodiment, a third secondary air supply valve 33 is additionally provided in addition to the first secondary air supply valve 27 and the second secondary air supply valve 28, and the discharge port of the air pump 26 is connected, via the conduit 30 and the third secondary air supply valve 33, to the exhaust pipe 22 arranged between the Nox absorbents 23, 24 and the oxidizing catalyst 20. The third secondary air supply valve 33 is connected to the input port 47 via the corresponding drive circuit 49, and the duty ratio of the third secondary air supply valve 30 is controlled based on the output signal of the electronic control unit 40. Thirdly, in this second embodiment, the second air-fuel ratio sensor 32 is arranged in the exhaust pipe 22 downstream of the secondary air supply port of the third secondary air supply valve 33 and upstream of the oxidizing catalyst 20. The most distinctive feature of the second embodiment resides in the fact that NOx is released from the NOx absorbents 23, 24 in accordance with necessity so that the absorbing operation of NOx can be carried out for a long time even if the NOx absorbents 23, 24 are not exchanged for new ones. However, in the second embodiment, the air-fuel ratio control carried out when the releasing operation of NOx is not carried out is basically the same as the control in the first embodiment. Namely, referring to Fig. 19 which schematically illustrates the air-fuel ratio A/F of the air-fuel mixture burned in the combustion chamber 3 and the air-fuel ratio A/F of the exhaust gas in the exhaust passage at the time after the warm-up of the engine is completed, also in this second embodiment, after the warm-up of the engine is completed, the air-fuel ratio A/F of the air-fuel mixture fed into the combustion chamber 3 is controlled by the feedback operation on the basis of the output signals of the first air-fuel ratio sensor 31 so as to be maintained between 14.3 and 14.5, as illustrated in Fig. 11A, and the amount of secondary air fed into the exhaust pipe 19 is controlled by the feedback operation on the basis of the output signals of the second air-fuel ratio sensor 32 so that the air-fuel ratio A/F of the exhaust gas flowing into the NOx absorbents 23, 24 and the oxidizing catalyst 20 is maintained between 15.1 and 15.3, as illustrated in Fig. 11B. Accordingly, also in this second embodiment, the purification rate of NOx by the three way catalyst 17 becomes almost 99.9 percent, and the purification rate of unburned HC and CO by the three way catalyst 17 becomes more than 80 percent. Thus, a small amount of unburned HC and CO is contained in the exhaust gas passing through the three way catalyst 17, but the amount of NOx contained in this exhaust gas becomes extremely small. In addition, since the amount of NOx contained in the exhaust gas passing through the three way catalyst 17 is extremely small as mentioned above, almost all NOx contained in this exhaust gas is absorbed in the NOx absorbents 23, 24. Furthermore, almost 100 percent of unburned HC and CO is purified in the oxidizing catalyst 20. Accordingly, also in the second embodiment, the amount of NOx and unburned HC and CO discharged into the outside air becomes almost zero. Referring to Fig. 20 which schematically illustrates the air-fuel ratio A/F of the air-fuel mixture burned in the combustion chamber 3 and the air-fuel ratio A/F of the exhaust gas in the exhaust passage at the time before the warm-up of the engine is completed, also in this second embodiment, at the time of the warm-up of the engine wherein the temperature of the engine is low, the air-fuel ratio A/F of the air-fuel mixture burned in the combustion chamber 3 is made a rich air-fuel ratio ranging from about 13.0 to about 14.0. Accordingly, at this time, the air-fuel ratio A/F of the exhaust gas discharged from the engine becomes a rich air-fuel ratio ranging from about 13.0 to about 14.0. In addition, at this time, to make the air-fuel ratio A/F of the exhaust gas flowing into the three way catalyst 17 a rich air-fuel ratio ranging from about 14.3 to about 14.5, secondary air is fed into the exhaust manifold 16 upstream of the three way catalyst 17, and secondary air is also fed into the exhaust pipe 19 so that the air-fuel ratio A/F of the exhaust gas flowing into the NOx absorbents 23, 24 and the oxidizing catalyst 20 becomes a lean air-fuel ratio ranging from about 15.1 to about 15.3. Figure 21 schematically illustrates the air-fuel ratio A/F of the air-fuel mixture burned in the combustion chamber 3 and the air-fuel ratio A/F of the exhaust gas in the exhaust passage in the case where NOx is released from the NOx absorbents 23, 24. As can be seen from Fig. 21, when NOx is to be released from the NOx absorbents 23, 24, the supply of secondary air into the exhaust manifold 16 upstream of the three way catalyst 17 is stopped, and the supply of secondary air into the exhaust pipe 19 upstream of the NOx absorbents 23, 24 is stopped. Accordingly, at this time, the air-fuel ratio A/F of the exhaust gas flowing into the NOx absorbents 23, 24 becomes rich, and thus NOx is released from the NOx absorbents 23, 24. Namely, at this time, a large amount of unburned HC and CO is contained in the exhaust gas passing through the three way catalyst 17, and this unburnt HC and CO react with the oxygen O 2 - on O 2- on the platinum Pt and are oxidized. Also, when the air-fuel ratio A/F of the exhaust gas flowing into the NOx absorbents 23, 24 becomes rich, the oxygen concentration in the exhaust gas is lowered considerably, and therefore the NO 2 is released from the absorbent. This NO 2 reacts with the unburnt HC and CO as shown in Fig. 7B and is reduced. In this way, when the NO 2 no longer exists on the surface of the platinum Pt, the NO 2 is successively released from the absorbent. Accordingly, when the air-fuel ratio of the exhaust gas is made rich, the NOx is released from the NOx absorbents 23, 24 in a short time. Namely, when the air-fuel ratio of the exhaust gas flowing into the NOx absorbents 23, 24 is made rich, first of all, the unburnt HC and CO immediately react with the O 2 - or O 2- on the platinum Pt and are oxidized, and subsequently if the unburnt HC and CO still remain even though the O 2 - or O 2- on the platinum Pt is consumed, the NOx released from the absorbent and the NOx discharged from the engine are reduced by these unburnt HC and CO. Accordingly, if the air-fuel ratio A/F of the exhaust gas flowing into the NOx absorbents 23, 24 is made rich, NOx absorbed in the NOx absorbents 23, 24 is released in a short time. In addition, since NOx thus released is reduced, there is no danger that NOx is discharged into the outside air. When the releasing operation of NOx from the NOx absorbents 23, 24 is carried out, secondary air is fed into the exhaust pipe 22. At this time, the amount of secondary air fed into the exhaust pipe 22 is controlled by the feedback operation on the basis of the output signal of the second air-fuel ratio sensor 32 so that the air-fuel ratio A/F of the exhaust gas flowing into the oxidizing catalyst 20 is maintained between 15.1 and 15.3. Accordingly, at this time, almost 100 percent of unburned HC and CO is purified in the oxidizing catalyst 20. Accordingly, even when NOx is released from the NOx absorbents 23, 24, there is no danger that NOx and unburned HC and CO are discharged into the outside air. Next, the control routine for the engine will be described with reference to Figs. 22 through 27. Referring to Figs. 22 and 23, in step 200, it is determined based on, for example, the temperature of the engine coolant whether or not the warm-up of the engine is completed. If the warm-up of the engine has been completed, the routine goes to step 201, and the first secondary air supply valve 27 is closed. Accordingly, the supply of secondary air into the exhaust manifold 16 is stopped. Then, in step 202, the fuel injection time TAU is calculated. The calculation routine of this fuel injection time TAU is shown in Fig. 24. Referring to Fig. 24, in step 220, it is determined whether or not the air-fuel ratio of exhaust gas A/F detected by the first air-fuel ratio sensor 31 is smaller than 14.5. If A/F ≧ 14.5, the routine goes to step 223, and a fixed value A is added to the correction coefficient FAFR. Then, the routine goes to step 224. Conversely, if A/F < 14.5, the routine goes to step 221, and it is determined whether or not the air-fuel ratio of exhaust gas A/F detected by the first air-fuel ratio sensor 31 is larger than 14.3. If A/F ≧ 14.3, the routine goes to step 222, and a fixed value B is subtracted from the correction coefficient FAFR. Then, the routine goes to step 224. Conversely, if A/F > 14.3, the routine goes to step 224. In step 224, the fuel injection time TAU is calculated based on the following formula. TAU = K•Q/N•FAFR Where, Q represents the mass flow amount of air detected by the mass flow detector 15, and N represents an engine speed. In addition, as mentioned earlier, K·Q/N represents a basic fuel injection time which is necessary to make the air-fuel ratio of air-fuel mixture the stoichiometric air-fuel ratio. As can be seen from Fig. 24, if A/F ≧ 14.5, the correction coefficient FAFR is increased and, if A/F ≦ 14.3, the correction coefficient FAFR is decreased. In addition, if 14.3 < A/F < 14.5, the correction coefficient FAFR is maintained as it is. Accordingly, the air-fuel ratio A/F of the air-fuel mixture fed into the combustion chamber 3 is maintained between 14.3 and 14.5. Turning to Fig. 22, if the calculation of the fuel injection time TAU is completed in step 202, the routine goes to step 203, and the duty ratio DUTY2 for controlling the second secondary air supply valve 28 is calculated. The calculation routine of this duty ratio DUTY2 is shown in Fig. 25. Referring to Fig. 25, in step 230, it is determined whether or not the air-fuel ratio of exhaust gas A/F detected by the second air-fuel ratio sensor 32 is larger than 15.1. If A/F ≦ 15.1, the routine goes to step 233, and a fixed value C is added to the duty ratio DUTY2. Conversely, if A/F > 15.1, the routine goes to step 231. In step 231, it is determined whether or not the air-fuel ratio of exhaust gas A/F detected by the second air-fuel ratio sensor 32 is smaller than 15.3. If A/F ≧ 15.3, the routine goes to step 232, and a fixed value D is subtracted from the duty ratio DUTY2. As can been seen from Fig. 25, when A/F ≦ 15.1, the duty ratio DUTY2 is increased, and thereby the amount of secondary air fed into the exhaust pipe 19 from the second secondary air supply valve 28 is increased. Conversely, when A/F ≧ 15.3, the duty ratio DUTY2 is decreased, and thereby the amount of secondary air fed into the exhaust pipe 19 from the second secondary air supply valve 28 is reduced. In addition, when 15.1 < A/F < 15.3, the duty ratio DUTY2 is maintained as it is. Thus, the air-fuel ratio A/F of the exhaust gas flowing into the NOx absorbents 23, 24 and the oxidizing catalyst 20 is maintained between 15.1 and 15.3. Turning to Fig. 22, if the duty ratio DUTY2 is calculated in step 203, the routine goes to step 204, and the ignition time  is calculated from the map illustrated in Fig. 17A. Then, in step 205, it is determined whether or not a fixed time has elapsed after the engine is started. If the fixed time has elapsed after the engine is started, the routine goes to step 207. Conversely, if the fixed time has not elapsed yet after the engine is started, the routine goes to step 206, and the ignition time  is retarded by α. Then, the routine goes to step 207. In step 207, the current engine speed NE is added to ΣNE. Accordingly, this ΣNE represents the cumulative value of the engine speed NE, and this cumulative value ΣNE is stored in the back up RAM45. Then, in step 208, it is determined whether or not the cumulative value ΣNE of the engine speed NE reaches the upper limit value MAX. If ΣNE ≦ MAX, it is thought that the absorbing ability of NOx of the NOx absorbents 23, 24 is not saturated and, at this time, the routine goes to step 209 in which the third secondary air supply valve 33 is closed. Then, the processing cycle is completed. When it is determined in step 200 that the warm-up of the engine is not completed, the routine goes to step 210, and the correction coefficient KR for the basic fuel injection time K·Q/N is calculated. This correction coefficient KR is a function of, for example, the temperature TW of the cooling water of the engine and is increased as the temperature TW of the cooling water of the engine becomes low, as illustrated in Fig. 17B. Then, in step 211, the fuel injection time TAU is calculated by multiplying the basis fuel injection time K·Q/N by the correction coefficient KR. At this time, the air-fuel ratio A/F of the air-fuel mixture fed into the combustion chamber 3 is made a rich air-fuel ratio corresponding to the value of the correction coefficient KR. Then, in step 212, it is determined whether or not the feedback control by the air-fuel ratio sensors 31, 32 becomes possible. If the feedback control by the air-fuel ratio sensors 31, 32 is not possible, the routine goes to step 213, and the duty ratio DUTY1 for controlling the first secondary air supply valve 27 is calculated. This duty ratio DUTY1 is a duty ratio which is necessary to bring the air-fuel ratio A/F of the exhaust gas flowing into the three way catalyst 17 to an air-fuel ratio ranging from about 14.3 to about 14.5, and this duty ratio DUTY1 is stored in advance in the ROM42 in the form of a map as illustrated in Figs. 17C as a function of, for example, the mass flow amount Q of air and the correction coefficient KR. Then, in step 214, the duty ratio DUTY2 for controlling the second secondary air supply valve 28 is calculated. This duty ratio DUTY2 is a duty ratio which is necessary to bring the air-fuel ratio A/F of the exhaust gas flowing into the NOx absorbents 23, 24 and the oxidizing catalyst 20 to an air-fuel ratio ranging from about 15.1 to about 15.3. This duty ratio DUTY2 is stored in advance in the ROM42 in the form of a map as illustrated in Fig. 17C as a function of, for example, the mass flow amount Q of air and the correction coefficient KR, or as a function of only the mass flow amount Q of air. When it is determined in step 212 that the feedback control by the air-fuel ratio sensors 31, 32 is possible, the routine goes to step 215, and the duty ratio DUTY1 for controlling the first secondary air supply valve 27 is calculated. The calculation routine of this duty ratio DUTY1 is shown in Fig. 26. Referring to Fig. 26, in step 240, it is determined whether or not the air-fuel ratio of exhaust A/F detected by the first air-fuel ratio sensor 31 is larger than 14.3. If A/F ≦ 14.3, the routine goes to step 243, and a fixed value E is added to the duty ratio DUTY1. Conversely, if A/F > 14.3, the routine goes to step 241, and it is determined whether or not the air-fuel ratio of exhaust gas A/F detected by the first air-fuel ratio sensor 31 is smaller than 14.5. If A/F ≧ 14.5, the routine goes to step 242, and a fixed value F is subtracted from the duty ratio DUTY1. As can been seen from Fig. 26, if A/F ≦ 14.3, the duty ratio DUTY1 is increased, and thereby, the amount of secondary air fed into the exhaust manifold 16 from the first secondary air supply valve 27 is increased. Conversely, if A/F ≧ 14.5, the duty ratio DUTY1 is decreased, and thereby, the amount of secondary air fed into the exhaust manifold 16 from the first secondary air supply valve 27 is reduced. In addition, if 14.3 < A/F < 14.5, the duty ratio DUTY1 is maintained as it is. Thus, the air-fuel ratio A/F of the exhaust gas flowing into the three way catalyst 17 is maintained between 14.3 and 14.5 Then, in step 203, the amount of secondary air fed from the second secondary air supply valve 28 is controlled by feedback on the basis of the output signal of the second air-fuel ratio sensor 32 so that the air-fuel ratio A/F of the exhaust gas flowing into the NOx absorbents 23, 24 and the oxidizing catalyst 20 is maintained between 15.1 and 15.3. When it is determined in step 208 that ΣNE > MAX, i.e., when the absorbing ability of NOx of the NOx absorbents 23, 24 approaches the saturated state, the routine goes to step 216, and the second secondary air supply valve 28 is closed. As a result, the air-fuel ratio A/F of the exhaust gas flowing into the NOx absorbents 23, 24 becomes rich, and thus the releasing operation of NOx from the NOx absorbents 23, 24 is started. Then, the routine goes to step 217, and the duty ratio DUTY3 for controlling the third secondary air supply valve 33 is calculated. The calculation routine of this duty ratio DUTY3 is shown in Fig. 27. Referring to Fig. 27, in step 250, it is determined whether or not the air-fuel ratio of exhaust gas A/F detected by the second air-fuel ratio sensor 32 is larger than 15.1. If A/F ≦ 15.1, the routine goes to step 253, and a fixed value C is added to the duty ratio DUTY3. Conversely, if A/F > 15.1, the routine goes to step 251. In step 251, it is determined whether or not the air-fuel ratio of exhaust gas A/F detected by the second air-fuel ratio sensor 32 is smaller than 15.3. If A/F ≧ 15.3, the routine goes to step 252, and a fixed value D is subtracted from the duty ratio DUTY3. As can been seen from Fig. 27, when A/F ≦ 15.1, the duty ratio DUTY3 is increased, and thereby the amount of secondary air fed into the exhaust pipe 22 from the third secondary air supply valve 33 is increased. Conversely, when A/F ≧ 15.3, the duty ratio DUTY3 is decreased, and thereby the amount of secondary air fed into the exhaust pipe 22 from the third secondary air supply valve 33 is reduced. In addition, when 15.1 < A/F < 15.3, the duty ratio DUTY3 is maintained as it is. Thus, the air-fuel ratio A/F of the exhaust gas flowing into the oxidizing catalyst 20 is maintained between 15.1 and 15.3. Turning to Fig. 23, if the duty ratio DUTY3 is calculated in step 217, the routine goes to step 218, and it is determined whether or not a fixed time has elapsed after the releasing operation of NOx is started. When a fixed time has elapsed after the releasing operation of NOx is started, i.e., when all NOx is released from the NOx absorbents 23, 24, the routine goes to step 219, and ΣNE is made zero. Then, the processing cycle is completed. In the next processing cycle, the supply control of secondary air by the second secondary air supply valve 28 is started again. According to the present invention, the amount of NOx discharged into the outside air can be reduced to almost zero. While the invention has been described by reference to specific embodiments chosen for purposes of illustration, it should be apparent that numerous modifications could be made thereto by those skilled in the art without departing from the basic concept and scope of the invention. An exhaust gas purification device comprising a three way catalyst, an oxidizing catalyst and a NOx absorbent which are arranged in this order in the exhaust passage. A rich air-fuel mixture is burned in the combustion chamber, and the exhaust gas discharged from the engine is initially introduced into the three way catalyst. Secondary air is fed into the exhaust passage between the three way catalyst and the oxidizing catalyst so that the air-fuel ratio of the exhaust gas flowing into the oxidizing catalyst and the NOx absorbent becomes lean.

Claims: An engine exhaust gas purification device comprising: air-fuel mixture control means for making an air-fuel mixture burned in a combustion chamber (3) of the engine (1) a rich air-fuel mixture; a three way catalyst (17) arranged in an exhaust passage of the engine (1); a NOx absorbent (23, 24) arranged in said exhaust passage downstream of said three way catalyst (17), said NOx absorbent (23, 24) absorbing NOx therein when an air-fuel ratio of exhaust gas flowing into said NOx absorbent (23, 24) is lean; characterized by a secondary air supply means (26, 28) for feeding secondary air into said exhaust passage upstream of the NOx absorbent between said three way catalyst (17) and said NOx absorbent (23, 24); and secondary control means for controlling an amount of secondary air fed from said secondary air supply means (26, 28) to maintain the air-fuel ratio of the exhaust gas flowing into said NOx absorbent (23, 24) at a lean air-fuel ratio. An exhaust gas purification device according to claim 1, wherein an air-fuel ratio of said rich air-fuel mixture is slightly smaller than the stoichiometric air-fuel ratio. An exhaust gas purification device according to claim 2, wherein the air-fuel ratio of said rich air-fuel mixture is between about 14.3 and about 14.5. An exhaust gas purification device according to claim 1, wherein said air-fuel mixture control means comprises an air-fuel ratio sensor arranged in said exhaust passage upstream of said three way catalyst and controls the air-fuel mixture burned in said combustion chamber on the basis of an output signal of said air-fuel ratio sensor to make an air-fuel ratio of said air-fuel mixture burned in said combustion chamber a target rich air-fuel ratio. An exhaust gas purification device according to claim 1, wherein said air-fuel ratio of the exhaust gas flowing into said NOx absorbent is slightly larger than the stoichiometric air-fuel ratio. An exhaust gas purification device according to claim 5, wherein said air-fuel ratio of the exhaust gas flowing into said NOx absorbent is between about 15.1 and about 15.3. An exhaust gas purification device according to claim 1, wherein said secondary air supply means comprises a secondary air inlet which is open to said exhaust passage, and said secondary air control means comprises an air-fuel ratio sensor arranged in said exhaust passage downstream of said secondary air inlet and controls the amount of secondary air fed from said secondary air supply means on the basis of an output signal of said air-fuel ratio sensor to maintain the air-fuel ratio of the exhaust gas flowing into said NOx absorbent at a target lean air-fuel ratio. An exhaust gas purification device according to claim 1, wherein said air-fuel mixture control means controls a richness of said air-fuel mixture burned in said combustion chamber to increase said richness of said air-fuel mixture when a warm-up operation is carried out, and air feeding means is provided for feeding secondary air into said exhaust passage upstream of said three way catalyst, said secondary air control means controlling a feeding operation of the secondary air fed from said air feeding means to feed the secondary air from said air feeding means when the warm-up operation of the engine is carried out and to stop the feeding operation of the secondary air fed from said air feeding means when the warm-up operation of the engine is completed. An exhaust gas purification device according to claim 8, wherein said secondary air control means controls an amount of the secondary air fed from said air feeding means to maintain an air-fuel ratio of exhaust gas flowing into said three way catalyst at a target rich air-fuel ratio when the warm-up operation of the engine is carried out. An exhaust gas purification device according to claim 9, wherein said target rich air-fuel ratio is between about 14.3 and about 14.5. An exhaust gas purification device according to claim 1, wherein ignition time control means is provided for controlling an ignition time to retard said ignition time for a predetermined time after the engine is started. An exhaust gas purification device according to claim 1, wherein said three way catalyst is an electrically heated catalyst and is electrically heated at least when the engine is started. An exhaust gas purification device according to claim 1, wherein said NOx absorbent comprises a first absorbent and a second absorbent arranged downstream of said first absorbent, and said first absorbent and said second absorbent have a different region of temperature at which an absorption rate of NOx becomes maximum. An exhaust gas purification device according to claim 13, wherein the region of temperature of said first catalyst is on the lower temperature side as compared with that of said second catalyst. An exhaust gas purification device according to claim 1, wherein said secondary air supply means comprises a secondary air inlet which is open to said exhaust passage, and an oxidizing catalyst is arranged in said exhaust passage between said secondary air inlet and said NOx absorbent, said secondary air control means controlling the amount of secondary air fed from said secondary air supply means to maintain the air-fuel ratio of the exhaust gas flowing into said oxidizing catalyst and said NOx absorbent at a lean air-fuel ratio. An exhaust gas purification device according to claim 15, wherein said oxidizing catalyst is an electrically heated catalyst and is electrically heated at least when the engine is started. An exhaust gas purification device according to claim 1, wherein an oxidizing catalyst is arranged in said exhaust passage downstream of said NOx absorbent. An exhaust gas purification device according to claim 17, wherein said oxidizing catalyst is an electrically heated catalyst and is electrically heated at least when the engine is started. An exhaust gas purification device according to claim 17, wherein said NOx absorbent releases absorbed NOx when the air-fuel ratio of the exhaust gas flowing into said NOx absorbent is made rich, and air feeding means is provided for feeding secondary air into said exhaust passage between said NOx absorbent and said oxidizing catalyst, said secondary air control means temporarily stopping a supply of secondary air fed from said secondary air supply means to make the air-fuel ratio of the exhaust gas flowing into said NOx absorbent a rich air-fuel ratio when NOx is to be released from said NOx absorbent, said secondary air control means controlling a feeding operation of secondary air fed from said air feeding means and temporarily carrying out said feeding operation of secondary air to make the air-fuel ratio of exhaust gas flowing into said oxidizing catalyst a lean air-fuel ratio when NOx is to be released from said NOx absorbent. An exhaust gas purification device according to claim 19, wherein said air feeding means comprises a secondary air inlet which is open to said exhaust passage, and said secondary air control means comprises an air-fuel ratio sensor arranged in said exhaust passage downstream of said secondary air inlet and controls the amount of secondary air fed from said air feeding means on the basis of an output signal of said air-fuel ratio sensor to maintain the air-fuel ratio of the exhaust gas flowing into said oxidizing catalyst at a target lean air-fuel ratio. An exhaust gas purification device according to claim 20, wherein said target lean air-fuel ratio is between about 15.1 and about 15.3.

Inventors: ITOU TAKAAKI, KUNITAKE KAZUHISA, OHNAKA HIDEMI, TAKEDA KEISO, TANAHASHI TOSHIO, TANAKA HIROSHI, ITOU, TAKAAKI, KUNITAKE, KAZUHISA, OHNAKA, HIDEMI, TAKEDA, KEISO, TANAHASHI, TOSHIO, TANAKA, HIROSHI, ITOU, TAKAAKI, C/O TOYOTA JIDOSHA K.K., KUNITAKE, KAZUHISA, C/O TOYOTA JIDOSHA K.K., OHNAKA, HIDEMI, C/O TOYOTA JIDOSHA K.K., TAKEDA, KEISO, C/O TOYOTA JIDOSHA K.K., TANAHASHI, TOSHIO, C/O TOYOTA JIDOSHA K.K., TANAKA, HIROSHI, C/O TOYOTA JIDOSHA K.K.
Applicants: TOYOTA MOTOR CO LTD, TOYOTA JIDOSHA KABUSHIKI KAISHA
Application year: 1994
Main classification: F01N3
IPC codes: F01N3, B01D53, F01N7, F02D43, F02D41, F02P5